The invention relates to a process for converting urea into ammonia.
In installations for the selective catalytic deNoxing (selective catalytic reduction, SCR) of flue gases, in particular of flue gases of a stationary power plant or a diesel engine, gaseous ammonia or an ammonia-water mixture is usually used as the reducing agent. Then, at a so-called SCR catalytic converter, the nitrogen oxides are also converted, in the presence of oxygen with the reducing agent ammonia, into nitrogen and water. There are strict safety regulations imposed on the transport, storage, and handling of ammonia. Urea, by contrast, is a relatively harmless substance. The technical problem of hydrolysis, i.e. of converting urea into ammonia, and the injection of the urea into the SCR reactor are factors which militate against the direct use of urea as reducing agent in a SCR reactor.
The use of urea in SCR reactors has hitherto only been known in the context of small installations, such as for example thermal power plant units or for diesel engines, in which the urea is injected directly into the SCR reactor generally with the aid of compressed air. For a system with a plurality of nozzles, the flow of urea needs to be regulated for each individual nozzle, which is technically complex.
The object of the present invention is to provide a method of converting urea to ammonia which helps overcome the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which provides ammonia for the deNoxing of flue gases in a form which is as simple as possible and is easy to handle from a safety aspect.
With the above and other objects in view there is provided, in accordance with the invention, a process for converting urea into ammonia, which comprises:
feeding an aqueous urea solution to a reaction chamber; flowing the aqueous urea solution through a catalytic converter in the reaction chamber and thereby catalytically converting urea into ammonia;
introducing additional energy into the reaction chamber by irradiating with microwaves; and
discharging an ammonia-water mixture formed in the reaction chamber from the reaction chamber.
In accordance with an added feature of the invention, the aqueous urea solution is conducted in the catalytic converter through capillary tubes containing a catalytically active substance. The active catalyst substance may be located on an inner surface of the tubes.
In accordance with an additional feature of the invention, the microwave irradiation is adjusted such that the ammonia evaporates together with water in the reaction chamber.
In accordance with a concomitant feature of the invention, the ammonia-water mixture is admixed as a reducing agent to a flue gas, and the mixture is fed to an SCR reactor together with the flue gas. There results a deNOxing reaction in the SCR catalyst so that the nitrogen oxides are removed from the flue gas.
In summary, the invention allows converting urea into ammonia, which can subsequently be used for deNOxing. The conversion is effected in a hydrolysis reactor using microwaves.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for converting urea into ammonia, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.